Asphalts have been used as sealants and paving compounds for thousands of years, on roofs, roads, and in many other applications. Originally they were taken straight from bituminous deposits in the earth and applied directly. More recently they have been derived from the distillation of petroleum products, typically being defined as the end residue of the distillation process.
For many years asphalts were used in the rough forms in which they were found or distilled, without additives. As a result of efforts to address the non-uniform composition and low melting or softening points of the distillate residues used as asphalt, however, additives began to be employed. It is now typical for raw asphalt residues to be blended with various additives into relatively uniform "flux" asphalts which may either be applied directly or used as base stock for more specialized or improved asphalt forms. Most typically, however, the resultant fluxes are modified even further before use, as they typically retain unacceptably low softening points --in the range of 85-90.degree. F. Given that surface temperatures on an asphalt roof or roadway may, under a summer sun, climb higher than 250.degree. F., it may be seen that in many applications softening points in the region of 90.degree. F. are unsuitable.
One of the most common means of raising the softening point of flux asphalts, and thereby increasing their utility, is the addition of oxygen to the flux. In a typical oxidation process, flux asphalt is placed in a large holding tank and heated to approximately 500.degree. F. at atmospheric pressure, and air is introduced at the bottom of the tank and allowed to percolate up toward the top of the asphalt, creating an exothermic chemical reaction which, depending upon the amount of oxygen added, has the effect of raising the softening point of the asphalt to more than 225.degree. F. The ASTM has published, in its D312-95a Standard Specification for Asphalt Used in Roofing, standards for the oxidization of asphalts, including a specification of physical properties of the asphalts following oxidization. ASTM standards from D312-95a are shown in Table 1. Type IV is the most highly oxidized of the ASTM asphalts, Type I the least.
TABLE 1 __________________________________________________________________________ Type I Type II Type III Type IV Property min max min max min max min max __________________________________________________________________________ Softening point (.degree. F.) 135 151 158 176 185 205 210 225 Flash point (.degree. F.) 500 -- 500 -- 500 -- 500 -- Penetration (units, 77.degree. F.) 18 60 18 40 15 35 12 25 Ductility (cm at 77.degree. C.) 10.0 -- 3.0 -- 2.5 -- 1.5 -- Solubility in trichloroethylene (%) 99 -- 99 -- 99 -- 99 -- __________________________________________________________________________
As shown in Table 1, however, an undesirable side effect of the oxidation process is a reduction in the ductility of the oxidized asphalt. The reduction in ductility results in a decreased resistance to thermal fatigue cycles, increased brittleness, and accelerated material breakdown. Thus further additives have been sought as a means of restoring, or even improving, flexibility and thermal resistance of oxidized asphalts. Beginning some years ago attempts were made to introduce various rubbers and rubber compounds to a wide variety of asphalt types. For example, U.S. Pat. No. 4,273,685to Marzocchi et al. discloses a rubber modified asphalt composition prepared by reacting a bituminous material with a polymerizable aromatic monomer and a rubbery polymer whereby the rubbery polymer is chemically integrated with the asphalt. U.S. Pat. Nos. 5,334,641and 5,525,653to Rouse disclose process for blending finely-ground recycled waste rubber at a -50 to -80 mesh particulate size with asphalt, while acknowledging that success in mixing asphalt with rubber is dependent upon the nature of the rubber used as well as its particulate size. Causyn et al., U.S. Pat. No. 5,436,285, discloses an improved paving composition comprising graded aggregate, asphalt, SBR polymer, and recycled crumb rubber. And Aoyama et al., U.S. Pat. No. 5,674,313, discloses a cationic-rubber modified asphalt emulsion with an organic coagulating agent.
It was early found, however, that while increasing the flexibility, and therefore the durability, of asphalts exposed to repeated heat-induced expansion and contraction cycles, the addition of rubber can substantially degrade the resistance of the asphalt mix to ultraviolet (UV) rays and attack by atmospheric ozone, thus again resulting in a mix which ultimately turns brittle and loses its sealing or paving attributes. Thus there exists and has for some time existed a need for an improved rubber additive --one which is more UV and ozone resistant, strong, and impervious to water, while retaining the ability to restore flexibility and durability to an oxidized asphalt. One of the most attractive candidates as such an additive has been styrene-ethylene-butylene-styrene block copolymer (SEBS). In comparison with other rubbers, and in particular with rubbers commonly used to modify asphalts, including styrene-butadiene-styrene (SBS) rubber, SEBS has been found to have superior UV and temperature-cycle damage resistance qualities, greater thermal stability, lower permeability to moisture, and improved resistance to attack by ozone, fats, and oils. But it has also been found that the introduction of SEBS rubber to asphalt presents special difficulties. Attempts to blend SEBS with asphalt in the manner used for other rubbers showed that the SEBS would not blend properly in such processes, particularly with oxidized asphalts. In particular, it has been found that attempting to modify oxidized asphalts with SEBS rubber according to prior art methods results in virtually complete failure. It appears that this is because the oxidation process itself drives off many of the more volatile components of the asphalt flux (commonly referred to as the "light end" components of the flux), which if present would act as solvents and/or catalysts to facilitate the breakdown or dissolution of the rubber compound and its chemical bonding with the asphalt to form a homogeneous modified asphalt product. With critical amounts of the lighter end materials driven off, the rubber cannot dissolve and bond with the asphalt in the required proportions, and instead remains largely separate in a rubber-asphalt suspension having undesirable service properties for sealing and paving.
Thus there exists a need for a fast, economical, and reliable process for effectively and thoroughly mixing SEBS rubber with asphalt, and in particular with oxidized asphalt, for use in roofing, sealing, paving, waterproofing membranes and other related technologies, in order to provide a strong, flexible, water-impervious, UV and ozone-resistant, durable asphalt having a high softening or melting point.